The ammonium-induced increase in rat brain lactate concentration is rapid and reversible and is compatible with trafficking and signaling roles for ammonium

• Published in: Journal of cerebral blood flow and metabolism 2007-11, Vol.27 (11), p.1830-1840
• Main Authors: Provent, Peggy, Kickler, Nils, Barbier, Emmanuel L, Bergerot, Astrid, Farion, Régine, Goury, Sarah, Marcaggi, Païkan, Segebarth, Christoph, Coles, Jonathan A
• Format: Article
• Language: English
• Subjects: Ammonium Chloride - pharmacology; Animals; Astrocytes - metabolism; Bioengineering; Biological and medical sciences; Brain Chemistry - drug effects; Cerebral circulation. Blood-brain barrier. Choroid plexus. Cerebrospinal fluid. Circumventricular organ. Meninges; Cerebrovascular Circulation - physiology; Data Interpretation, Statistical; Electrodes, Implanted; Extracellular Space - drug effects; Extracellular Space - metabolism; Fundamental and applied biological sciences. Psychology; Glutamic Acid - isolation & purification; Glutamic Acid - metabolism; Homeostasis - physiology; Image Processing, Computer-Assisted; Imaging; Lactic Acid - metabolism; Life Sciences; Magnetic Resonance Spectroscopy; Male; Medical sciences; Neurology; Neurons - metabolism; Neuropharmacology; Neuroprotective agent; Pharmacology. Drug treatments; Potassium - metabolism; Quaternary Ammonium Compounds - pharmacology; Rats; Rats, Wistar; Signal Transduction - drug effects; Signal Transduction - physiology; Sodium Chloride - pharmacology; Vascular diseases and vascular malformations of the nervous system; Vascular Resistance - drug effects; Vascular Resistance - physiology; Vasodilation - physiology; Vertebrates: nervous system and sense organs
• ISSN: 0271-678X; 1559-7016
• DOI: 10.1038/sj.jcbfm.9600480

The glutamate—glutamine shuttle requires a flux of fixed N from neurons to astrocytes. The suggestion that some or all of this N is ammonium has received support from reports that ammonium (as NH+4) rapidly enters astrocytes. Ammonium might also help control astrocyte energy metabolism by increasing lactate production. If ammonium has these functions, then its effect on brain metabolism must be rapid and reversible. To make a minimal test of this requirement, we have followed the time courses of the changes induced by a 4 min venous infusion of 1 mol/L NH4Cl, 2.5 mmol/kg body weight, in rat. Extracellular [NH+4] in cortex, monitored with ion-selective microelectrodes, reached a peak of approximately 0.7 mmol/L 1.65 mins after the end of the infusion, then recovered. Brain metabolites were monitored non-invasively every 4 mins by 1H magnetic resonance spectroscopy. Lactate peak area during the 3.2 min acquisition starting at the end of the infusion was 1.84 ± 0.24 times baseline (± s.e.m., P = 0.009, n = 9). Lactate increased until 13.2 ± 2.1 mins after the end of the infusion and recovered halfway to baseline by 31.2 mins. Glutamate decreased by at least 7.1% (P = 0.0026). Infusion of NaCl caused no change in lactate signal. Cerebral blood flow, measured by arterial magnetization labeling, more than doubled, suggesting that the lactate increase was not caused by hypoxia. At least three consecutive ammonium-induced increases in lactate signal could be evoked. The results are compatible with an intercellular trafficking/signaling function for ammonium.